Conventional time-of-flight (ToF) cameras contain an active light-source that emits amplitude-modulated light (e.g., strobed light) that illuminates a scene. The optical signal returning to the camera sensor exhibits a shift in phase corresponding to the propagation distance of the signal, which allows object depth to be calculated. Thus, conventional time-of-flight (ToF) cameras rely on phase-sensing to determine object depths.
Unfortunately, conventional ToF cameras that measure phase shift in order to determine object depth (“phase-ToF cameras”) suffer from at least three technological problems:
A first technological problem with conventional phase-ToF cameras is that phase is an ambiguous indicator of depth because of “phase-wrapping”. The phenomenon of phase-wrapping occurs because phase is periodic and a modulo value: thus, the same phase will be observed whenever the optical path length is equal to a given distance plus an integer multiple of the wavelength of the amplitude modulation. For example, the same phase will be observed when the optical path length is equal to (i) a given distance, (ii) the given distance plus the wavelength of the amplitude modulation, or (iii) the given distance plus twice the wavelength of the amplitude modulation,
A second technological problem with conventional phase-ToF sensors is multi-path interference. The phenomenon of multi-path interference (“MPI”) occurs when a sensor pixel simultaneously receives light that has traveled different optical path lengths before reaching the pixel. MPI may occur, for example, if a pixel simultaneously receives both: (a) light that reflects multiple times before reaching the pixel (and thus travels a longer path) and; and (b) light that reflects only once before reaching the pixel (and thus travels a shorter path). Because light path distance affects the phase of light received by the ToF sensor, light that reflects multiple times before reaching the sensor tends to have a different phase at the sensor than that of light that reflects only once. The camera pixel measures the sum (superposition) of all light that it is receiving, and thus tends to measure a corrupted phase that is not the phase of the single-bounce light. As a result, multi-path interference tends to corrupt the object depth measured by a phase-ToF camera.
A third technological problem with conventional phase-ToF sensors is that they tend to be inaccurate when the signal-to-noise (SNR) is low.